Trisomy 21(down syndrome)?

Question

For my science class i have to learn about chromosomes and i need to know..

how people become down sydrome.

Sometimes this division does not happen properly and one cell may contain 22 chromosomes and the other may contain 24 chromosomes. This can happen if the chromosomes do not properly separate and instead "stick together."

can someone explain how this works..

Answer 1

Trisomy 21

Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete

Answer 2

The previous poster has it mostly correct (although the 47-XX is not right; Down syndrome people can be XY as well), but you may not understand the jargon.

"Meiosis" is the process by which gametes (sperm and egg cells) are formed--this is how we go from 46 chromosomes to 23, which re-combine in the zygote (the fertilized egg) so that there should be 46 total again.  Nondisjunction is that "sticking together" thing you're talking about.  No one is quite sure how it happens, but it happens much more often in eggs produced by older women.  That is why more advanced age in the mother is considered to be a risk factor for Down syndrome.  

In any case, when it does happen, you described the results very well.  You will have a gamete with 24 and another with 22 chromosomes, and either of those things will cause a problem if that gamete is involved in fertilization.  If it's the one with 24, you have a trisomy because there will be three of that particular chromosome in the fetus; if it's the one with 22, it's called a unisomy (and generally that fetus will not develop past the first couple of weeks unless the unisomy is of the s*x chromosomes--you can live with that).

Nondisjunction and trisomy can happen with any pair of chromosomes, but the reason that we generally only see it in people as Trisomy 21 is that in almost all cases, a trisomy of any other chromosome will result in a nonviable fetus--one that does not live to be born.  (There are a few exceptions to this.  Some people do survive with trisomies of chromosomes 13 and 18, which are called Patau and Edward syndromes respectively, but which are much more severe than Down syndrome and very few of those people survive more than a few years.  You can also have a trisomy of the s*x chromosomes--the X and Y chromosomes--with relatively little pathology.)  Most Down syndrome is caused by this kind of nondisjunction.

There is a rare form of Down syndrome that is caused by something known as a Robertsonian translocation--that is mostly what that extra 5% is in the previous answer.  In this, most of a chromosome 21 becomes attached to another chromosome (usually 14 or 15).  This doesn't cause Down syndrome in the first carrier, but when that person has children, they are more likely to inherit an imbalanced assortment of chromosomes (because they will get that Robertsonian chromosome with an extra arm plus the normal chromosome 21).  

But that may be more than you wanted to know.  ;-)